Despite several past and present missions to Mars, very little information is available on its subsurface outside of its polar caps and its very superficial layer.
■ Despite several past and present missions to Mars, very little information is available on its subsurface outside of its polar caps and its very superficial layer. One of the scientific objectives of the ESA’s ExoMars mission is to characterize the water/geochemical environment as a function of depth and investigate the planet subsurface to better understand the evolution and habitability of the planet. The electromagnetic survey of subsurface will provide a nondestructive way to probe the subsurface and look for potential deep liquid water reservoirs. ■ In the frame of this spatial mission, the LATMOS is currently developing a ground penetrating radar (GPR) called EISS “Electromagnetic Investigation of the Sub Surface”, which is a enhanced version of Netlander’s GPR (mission cancelled in 2004). ■ The GPR main objective is to perform sounding of the Martian sub-surface down to kilometric depth from the surface. Because the current conditions of pressure (~6.1mbar) and temperature (Tmoy = -63°C) on Mars prohibit the presence of liquid water on its surface. However, the presence of paleo-hydrological structures suggests that water flowed on Mars as following photography of old river valleys. (Viking, Mars Orbiter, ...).
Coupling between the sub-surface and the HF monopoles
Identification of received echoes (bi-static mode)
EISS: Impulse HF Ground Penetrating Radar
■ To keep the mass and volume of the antenna within reasonable limits, loaded dipole, composed of two identical monopoles is used. The resistive profile of the antenna follows a Wu-King profile which is optimized to transmit the pulse without noticeable distortion and avoid ringing phenomenon. The downside of the design is the low efficiency of such an antenna (only a few percents) because of the power that is dissipated into the resistors. The resistive profile of each monopole must be chosen in order: to ensure that the current intensity at the end of the monopole is null over the whole bandwidth: it guarantees a progressive wave traveling without distortion along the antenna with no reflection at the end. to obtain an antenna impedance as flat as possible over the whole frequency bandwidth: each monopole impedance needs to be matched to the electronics impedance to optimize the signal transmission and optimize thus instrument efficiency.
The propagation delay for the main waves is given by: - Direct wave travelling in the air: τd - Ground wave travelling in the layer: τg - Waves reflected n times: τrn
■ The following results will focus on the performances of the antenna (decrease of the current along the antenna and measured impedance) obtained when the antenna is deployed on the surface thus the interface between the two media: vaccum and homogeneous sub-surfaces with different relative permittivity values. The antenna behaves as if it were surrounded by a medium having the following electrical properties equal to the arithmetic average.
τd
d = c
d εr τg = c
d2 + (2nh)2 εr τrn = c
■ Radargram showing the envelope of the received signal for the Hz component as a function of the distance between the transmitter and the receiver. The homogeneous layer is 500m thick and has a relative permittivity of εr1=3. At the bottom of this layer, a perfectly reflecting horizontal layer is placed. The direction between the transmitter and receiver makes an angle of 45° with respect to the direction of the transmitting electrical antenna.
■ The exact characteristics of the Martian subsurface at the landing site are not a priori known values but a relative permittivity value εrs around 4 seems a realistic one. To obtain the best performances, the resistive profile should be optimized according to the geolectrical properties of the sub-surface εrs(relative permittivity) et σs (conductivity). Three different resistive profiles have be considered: profile 1: optimized for the vacuum profile 2: optimized for a sub-surface with a relative permittivity of εrs=4 profile 3: optimized for a sub-surface with a relative permittivity of εrs=7
Propagation delay
Passive measurement
d (Lander-Rover) [m]
Measurements of the antenna impedance
Impact of the angle between the two monopoles of the HF antenna (bistatic mode) Direct wave Hx
Hy
on the sand of the forest of Fontainebleau (FR)
Reflected wave once Hz
Hx
Hy
Hz
Retrieval of the top layer permittivity and conductivity value
for the 3 studied profiles (FDTD simulations)
Antenna impedance over the whole frequency bandwidth: Simulations run for different permittivity values on the sub-surface characteristics show that there is a coupling between the antenna and the sub-surface top layer and that it has have an impact on the antenna effective impedance. The sub-surface impedance is a decreasing function of its own permittivity er. Simulations show that the real parts of the measured impedance is not constant over the whole frequency range and that the obtained variations with frequency depend on the pair subsurface permittivity value – resistive profile. The best matching can be obtained for an impedance as flat as possible over the whole band width: profile 2.
Retrieval of the top layer conductivity value: The estimation of the top layer conductivity from impedance measurements remains difficult, but possible with the study of the values measured at very low frequencies (
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